1. Field
Example embodiments relate to a semiconductor device of which a residual stress and pit density are decreased.
2. Description of the Related Art
Sapphire is frequently used as a substrate to form a nitride-based semiconductor device. However, it is difficult to manufacture a chip with sapphire since a sapphire substrate is expensive and solid and has low electroconductivity. Also, when the sapphire substrate is epitaxially grown to a large size, the sapphire substrate bends in a high temperature environment due to its low thermal conductivity, and thus, it is difficult to manufacture a large sapphire substrate. In order to overcome the aforementioned limit, a nitride-based semiconductor device using a silicon substrate instead of the sapphire substrate has been developed. Since the silicon substrate has a higher thermal conductivity than that of the sapphire substrate, the silicon substrate does not significantly bend at a high temperature at which a nitride thin film grows, so it is possible to grow the silicon substrate to a large size.
When a nitride-based semiconductor device, e.g., GaN, grows on the silicon substrate, a thermal tensile stress is incurred on a GaN layer during a cooling process due to a difference between thermal expansion coefficients. If the thermal tensile stress exceeds a threshold value (˜400 MPa), cracks occur. Thus, in order to prevent cracks by controlling a stress incurred during growth and cooling processes, several types of buffer layers or interlayers are used, or a patterned growth method is used. Also, since a threading dislocation of about 109˜1010 cm−2 is incurred due to a lattice mismatch between the silicon substrate and the GaN layer, in order to prevent this lattice mismatch, a proposal has been made to decrease a dislocation density of a nGaN clad that is a template having an active layer deposited thereon.